Foundation requirements and shallow foundation types

Foundation

A foundation is the lowest part of a structure, usually below ground, which transmits the load of the structure to the soil or rock.

Basic requirements of a foundation

1. Adequate depth, below the zone of frost, shrinkage, scour and the loose top soil.
2. Safe against shear failure: Soil pressure within the safe bearing capacity.
3. Settlement within limits, and uniform (no harmful differential settlement).
4. Safe against sliding, overturning and uplift.
5. Durable: Resistant to chemicals, sulphates and moisture.
6. Resists earthquakes and other special loads.
7. Economical and practical to construct.
8. Level and stable base: Bottom level on firm soil, with good drainage.

Types of shallow foundation

1. Strip / wall footing: Continuous below a load-bearing wall; it spreads the load by projecting offsets (in masonry, the width is about twice the wall width).
2. Isolated (spread) footing: A separate pad under each column, stepped or sloped, of reinforced concrete.
3. Combined footing: One footing for two or more columns, rectangular or trapezoidal, used when columns are close together or near the boundary.
4. Strap footing: Two footings joined by a strap beam to take an eccentric column load.
5. Raft (mat) foundation: A large slab covering the whole plan, for weak soil and heavy loads.
6. Grillage footing: Layers of steel beams in concrete, for heavy loads.

  Strip:  |wall|      Isolated: |col|      Raft: ___________
          [____]                [___]            |_________|
  Combined: |c1|   |c2|          Strap: [F1]==beam==[F2]
            [_________]